a) Field of the Invention
The invention concerns additives which may be used as lamination lubricating agents or are part of lamination lubricating agents. The invention also concerns compositions including these additives and which may be used in the lamination of a sheet such as lithium in order to obtain thin films, which may be used as such in the production of polymer electrolyte electrochemical cells. In addition, the invention concerns the use of the additives per se or compositions containing same to provide, by lamination, films of alkali metals or alloys thereof which may be used as anodes in electrochemical cells preferably with polymer electrolytes. The invention also concerns a process of lamination utilizing these additives or compositions containing same as lamination lubricating agents.
b) Description of Prior Art
The production of thin films of lithium having thicknesses lower that 75 micrometers and in the form of wide bands, for example 5 centimeters or more and in lengths of many tens of meters, by means of rapid and reliable processes, faces important technical difficulties which are attributable to the extreme physical and chemical properties of this metal: chemical reactivity, malleability, rapid self-welding by simple contact and strong adhesion on most solid materials, for example the usual metals.
This difficulty is confirmed by the difficulty of obtaining from suppliers of specialty metals and chemical products, thin lithium films 40 micrometers (xcexcm) thick and less, of sufficient surface and length, having an adequate surface finish and chemical property to be used in lithium cells.
Presently, cold extrusion is used for the continuous production of sheets 75 xcexcm and more. These thicknesses are generally adapted to the production of lithium cells utilizing liquid electrolytes. For lower thicknesses, the films obtained by extrusion are thereafter laminated between rollers made of hard materials. These processes have been described and are commercially used for the production of limited quantities of sheets of 30-75 microns. Reference will particularly be made to U.S. Pat. No. 3,721,113, inventor Hovsepian and dated Mar. 20, 1973. Many successive passes, according to the present state of the art, are required to give films 40-30 xcexcm.
Other alternative processes have been described to give ultra-thin sheets, which are used for example in the production of polymer electrolyte cells in the form of thin films. This is the case for example of a lamination process between steel rollers which are protected by films of hard plastic which are non reactive towards lithium, such as described in U.S. Pat. No. 3,721,113, or of processes based on the coating of molten lithium on a metallic of plastic support, described in U.S. Pat. No. 4,824,746, inventors Andrxc3xa9 Bxc3xa9langer, et al, dated Apr. 25, 1989.
The difficulty in achieving the lamination of lithium to thicknesses which vary between 40 and 5 microns for the production of polymer electrolyte cells is mainly due to the reactivity and the adhesion of the laminated metal with the materials with which it is in contact: lamination rollers, protection plastic films, lamination additives, as well as to the bad mechanical properties of thin sheets. For examples a film of lithium 20 xcexcm thick and 10 cm wide breaks under a drawing tension higher than 579.13 KPa which does not permit to pull on the film which exits from the laminating machine or to release it from the lamination rollers if lithium adheres somewhat thereto.
An approach which is normally used for the extensive lamination or calandering of hard metals, such as iron and nickel, is based on the use of liquid lamination additives consisting of organic solvents which may contain greases or lubricating agents. Examples include fatty acids or derivatives thereof such as for example lauric or stearic acids and alcohols, for example the compounds known under the trade marks EPAL 1012 of Ethyl Corporation U.S.A., which are mixtures of primary linear C10-C12 alcohols.
For lithium and particularly for lithium intended for electrochemical cells, the use of such additives involves two major difficulties:
1) the chemical reactivity of lithium which is in contact with solvents or lubricating agents including reactive organic functions, such as organic acids and alcohols. These functions react at the surface of lithium during and after lamination and create passivation films at the surface of the metal. This is harmful for a good operation of electrochemical cells especially when the latter are intended to be rechargeable;
2) the difficulty of removing the lubricating agents or greases which are in contact with lithium after lamination. This is the case, for example, when lubricating agents which mostly consist of hydrocarbon chains are selected, because they are nearly not reactive with lithium. These compounds constitute electrical insulating materials which are harmful to the good operation of lithium electrodes made with these sheets. Such lubricating agents are not very soluble in polymer electrolytes and should therefore be removed from the surface of lithium by washing after lamination. In addition to the fact that the washing of the surface of lithium is a delicate and costly operation, it will be noted that this operation inevitably contributes to contaminate the surface of lithium, in spite of all the care which may be used to control the quality of the surface of the metal. The latter reacts indeed irreversibly with all the impurities, including water, which are present in the washing solvents, or resulting from accidental contaminations.
It can be shown that the lithium obtained after a process of lamination with an additive followed by a subsequent washing is generally more contaminated at the surface than a lithium which is laminated without additive. This phenomenon may be observed with optical means, including a simple visual inspection or by a control of the impedance of the electrochemical batteries produced with polymer electrolytes. On the other hand, lamination without solvent and without lubricating agent means low production speeds and a tendency of the fresh lithium to stick to the rollers or the protection films of the rollers; moreover, many consecutive laminations are therefore required to reach thicknesses of the sheet lower than 40 micrometers.
It is an object of the present invention to solve the problem of lamination or calandering of lithium films, to thicknesses between 40 and 5 xcexcm, which can be directly used in lithium batteries made with thin films, for example polymer electrolyte batteries.
It is also an object of the invention to propose lubricating additives which are chemically compatible with lithium and which may be used in a process of lamination which does not require a subsequent washing of the surface of laminated lithium.
Another object of the invention resides in a composition consisting of a lamination lubricating agent including an appropriate solvent as well as an additive having two functions.
Another object of the invention resides in an improvement of the process of lamination of lithium in the presence of an improved lubricating agent.
Another object of the invention is to propose lamination lubricating additives enabling to produce in a single pass, extremely thin lithium, for example a thickness lower than 10 xcexcm, at appreciable speed which may be up to 50 m/min., and even more, and with an excellent control of the surface properties: uniform surface profile and low impedance of the passivation layer when the sheets thus produced are used in an electrochemical cell.
Another object of the invention consists in the provision of a lamination lubricating agent including an additive and solvents, in which the latter are selected for their chemical compatibility with a lithium which is intended for an electrochemical cell.
As used in the present description and in the appended claims, chemical compatibility of solvent or of an additive toward lithium of an electrochemical generator means the absence of chemical reaction with lithium or also, a limited chemical reaction leading to the formation of a passivation film which is not harmful to electrochemical exchanges at the interface lithium/electrolyte of said cell.
Another object of the invention resides in the chemical formulation of a lubricating agent for use in lamination which is not volatile and is selected so that it may be kept at the surface of lithium after lamination and this without harming the good operation of the sheet of lithium (anode), when the latter is used as such in an electrochemical cell, i.e. without any previous washing step.
Another object of the invention resides in an improved process of lamination utilizing the additives according to the present invention.
The invention is based on the choice of a lubricating chemical compound of high molecular weight including at least two segments of different chemical nature: a chain or a chain segment having a lubricating function (L) as made, for example, of a hydrocarbon chain including at least 8 carbon atoms associated with a solvating chain (S), capable of ionically dissociating at least in part a metallic salt, for example of lithium, such as a chain segment of ethylene polyoxide. The solvating segment present in the lubricating additive is selected so as to confer an ionic conductivity to the lubricating additive.
A preferred but non limiting manner of inducing ionic conductivity in the lubricating additive is obtained when the laminated lithium is contacted with the electrolyte (solvating polymer+lithium salt) of the cell. The salt present in the electrolyte is then diffused in the solvating part of the additive and locally constitutes a complex conductor (solvating chain+salt).
The lubricating agent according to the invention comprises at least one sequence:
Lxe2x80x94Yxe2x80x94S
where:
L designates a hydrocarbon radical, such as alkyl, alkylene, linear or cyclic or aryl-alkyl, saturated or non saturated, preferably containing more than 8 carbon atoms used as a lubricating segment which is compatible with lithium;
S designates an oligomer segment including heteroatoms such as O or N, and capable of solvating salts, for example salts of lithium and ensuring an electrolytic conductivity;
Y designates a chemical bond or a chemical group which is at least divalent joining the chains or chain segments L and S.
The solvating cell segment S may be joined to a terminal group C to constitute the sequence Lxe2x80x94Yxe2x80x94Sxe2x80x94C, C then being selected for its low reactivity with lithium.
C may for example designate a group Yxe2x80x2xe2x80x94Lxe2x80x2, which is identical or different from group Yxe2x80x94L, an alkyl radical, an alkyl-aryl radical, of valence equal to or higher than 1. According to a variant, C is a polymerisable group which can be incorporated to at least one of the repetitive units which constitute the polymer electrolyte of an electrochemical cell. According to another variant, C includes a ionophoric group which is somewhat dissociable and is capable of inducing an intrinsic ionic conductivity in the additive.
Examples of polymeric solvating chains are given in the following patents: U.S. Pat. No. 4,303,748, inventors Michel Armand, et al, dated Dec. 1, 1981, and U.S. Pat. No. 4,578,326, inventors Michel Armand, et al, dated Mar. 25, 1986. Chains bases on ethylene oxide xe2x80x94[CH2xe2x80x94CH2xe2x80x94O]nxe2x80x94, propylene oxide xe2x80x94[CH2xe2x80x94CH2(CH3)xe2x80x94O]nxe2x80x94 or on poly-(N-methyl-ethylene-imine) xe2x80x94[CH2xe2x80x94CH2xe2x80x94N(CH3)]n or their combinations are generally preferred, but other solvating functions may also be used as long as they may induce an ionic conductivity in the lubricating additive.
In the case where the hydrocarbon segment originates from a fatty acid, the bond Y preferably consists of ester (L)xe2x80x94COxe2x80x94Oxe2x80x94(S) or ether (L)xe2x80x94Oxe2x80x94(S) groups. Y may also represent amine or amide groups.
According to a preferred embodiment of the invention, the segment may correspond to the hydrocarbon chain of a fatty acid including at least 8 and preferably from 10 to 30 carbon atoms. For example, L may consist of a hydrocarbon chain of a fatty acid such as stearic acid and Y may then be a chemical bond of the ester or ether type, or may represent a carboxylate group which originates from a fatty acid ester.
According to another preferred embodiment of the invention, the segment S may consist of polyethers or polyamines of molecular weights 150.
According to another preferred embodiment of the invention, the terminal group C may also include a chemical function capable of covalently fixing a metallic salt, for example a lithium salt.
According to another preferred embodiment of the invention, the chemical bond C may include a lithium salt which is chemically grafted by the anion or by means of one or more in saturations.
The invention also resides in a lithium film covered with a thin layer of the additive defined above, the thickness of the film being between 5 and 50 microns.
Another aspect of the invention concerns a lithium based anode prepared from a sheet of lithium covered with a thin layer of the additive defined above, the thickness of the anode being between 5 and 50 xcexcm, which is in direct contact with a sheet including carbon or metals capable of chemically forming an alloy of lithium or an intercalating compound of lithium.
The invention also concerns a polymer electrolyte electrochemical cell including a lithium anode which is prepared as indicated above, in which a free lithium salt is present in the electrolyte so as to form, by diffusion, a complex electrolyte conductor with the chain S of the additive, and the latter may be soluble in the electrolyte.
According to another embodiment of the invention, there is provided the use of an additive or a composition as defined above for producing films of alkali metals or alloys thereof by lamination, which may be used as anodes in polymer electrolyte electrochemical cells.
The invention finally concerns a process of lamination which is intended to give thin films of alkali metals or alloys thereof, from a sheet of said metals or alloys thereof wherein the sheet is passed between working rollers with a laminating lubricating agent to laminate the sheet into thin films, characterized in that the lubricating agent includes an additive or a composition as defined above.
A particularly interesting additive is a polyoxyethylene distearate whose solvating segment corresponds to a molecular weight between about 150 and 4000.
The compositions according to the invention preferably contain 0.01 to 10% by weight of additive, more specifically about 0.2%. With respect to the solvent, it may be selected among saturated or partially saturated linear, cyclic or aromatic hydrocarbons, for example heptane, benzene, toluene, cyclohexane or a mixture thereof. It may also be selected among aprotic solvents which are compatible with lithium.
A particularly advantageous formulation consists in using a family of compounds of the type: Lxe2x80x94Yxe2x80x94Sxe2x80x94Yxe2x80x94L based on diesters of fatty acids, for example,
CH3xe2x80x94(CH2)16xe2x80x94COOxe2x80x94(CH2xe2x80x94CH2xe2x80x94O)nxe2x80x94OOC(CH2)16xe2x80x94CH3
where n preferably varies between 3 and 100. Compounds including polyether segments of molecular weight equal to 200, 400 and 600 are commercially available from Polyscience, preferably POE 400 Aldrich No 30541-3.
The stearate segments have excellent lubricating properties and their hydrocarbon chains are inert towards lithium; in this case, the bond Y is ensured by the carboxylic group of the starting fatty acid. The terminal group C then consists of a segment Yxe2x80x2xe2x80x94Lxe2x80x2 identical to Lxe2x80x94Y.
It has been observed that a central polyether chain, of low molecular weight, distearate POE 200, is sufficient to give to the lubricating compounds an ionic conductivity of the order to 1xc3x9710xe2x88x925xe2x88x92 S.cm at ambient temperature when a lithium salt such as Li(CF3SO2)2NLi is added in a ratio such that the ratio O/Li is 30/1. This value is amply sufficient to ensure ionic exchanges at the lithium/electrolyte interface of an electrochemical cell taking into account the small thickness of the residual deposit of the lubricating agent after lamination.
These preferred formulations are given by way of example of possible embodiments of the invention. Other lubricating and solvating functions L and S may be used as well as other bonds Y. By way of non limiting example, reference may be made to the following articles which deal with types of solvating chains:
Polymer Electrolytes review-1, J. R. MacCallum and C. A. Vincent eds. Elsevier Applied Science London (1987);
Polymer Electrolyte, review 2, J. R. MacCallum and C. A. Vincent eds. Elsevier Applied Science London (1989);
Solid Polymer Electrolytes, F. M. Gray VCH Publisher New-York, Weinheim (1991); as well as
Surface Active Ethylene Oxide Adducts, by V. Schoenfeldt-Permagon Press, (1966).
The preparation of the additives according to the present invention is well known to one skilled in the art and needs no detailed discussion in the present context. It is sufficient to mention that any skilled chemist would have no problem to synthesize the desired additive once the solvating and lubricating chains are established and the choice of the chemical bond which is intended to be used has been made.
During lamination, it is generally preferable to dilute the lubricanting agents according to the invention in one or more solvents which are compatible with lithium and which are preferably linear, saturated or partially unsaturated, or cyclic aromatic hydrocarbons such as heptane, benzene, toluene, cyclohexane or any other pre-dehydrated aprotic organic solvent or a mixture thereof. This dilution enables to reduce to a minimum the required quantity of lubricant and to obtain optimum qualities of lithium for use in an electrochemical cell. These solvents are previously dehydrated, for example on a molecular sieve, to lower the water content below 100 ppm. The concentrations of additives may vary up to about 10% by weight for example between 0.01 and 10% by weight, preferably 0.2% by weight. The addition of the lubricanting agent in solution is carried out in a controlled manner immediately before lamination between rollers. The laminated film is dried by a continuous operation with dry air immediately at the outlet of the rollers and is thereafter wound with or without a separator film of inert plastic, preferably of propylene or polyethylene.